An analog or digital signal optical communication system typically uses a semiconductor injection laser whose optical output is modulated by varying the current injected into the laser (current injection modulation) in accordance with a modulating signal which represents information to be transmitted. The output is then transmitted over a transmission line, typically an optical fiber, to a receiver where the modulating signal is detected and the information is recovered and utilized. For example, such a system is usable in cable TV, satellite communication, and radar communication.
In order to make economical use of such a system, many different signals are simultaneously transmitted (multiplexed) over the same transmission line using the same laser. For this purpose, the laser typically is modulated by a plurality of subcarrier frequencies (frequency multiplexing) which are themselves either amplitude or frequency modulated (AM or FM) by a corresponding plurality of signals. However, either the inherent dispersive nonlinearity (if any) of response of the laser material to these electrical signals or the inherent nonlinearity of the response due to the nonlinear interferometric properties of the laser cavity, or both, gives rise to a resulting unavoidable overall nonlinearity of response of the laser. This overall nonlinearity in the laser response to electrical signals results in a generation of undesirable harmonics of the signals. Furthermore, wavelength chirping of the modulated laser, coupled with the non-linear transmission characteristics of any dispersive element located along the transmission line, will also give rise to harmonic distortion and hence undesirable intermodulation between the different frequency-multiplexed signals, whereby unwanted harmonic distortion and unwanted intermodulation distortion (cross-talk) occur, respectively.
In prior art, in order to minimize the aforementioned undesirable distortions, workers in the art have selected the operating DC bias current of the laser such that, when the modulating signals are all zero, the laser operates at the center of its most nearly linear range (region)--i.e., the region where the intensity of optical output of the laser is most nearly a linear function of current applied to the laser (maximum linearity region). To this end, various electrical feedback schemes have been taught to ensure that the laser operates with a DC bias current in this linear range, so that the second harmonic distortion is minimized. That is, for example, by means of filters and feedback the second harmonic in the optical output of the laser (in response to a test signal applied to the laser) is detected and a correction signal for the DC bias current is fed back to adjust the DC bias current to minimize this second harmonic distortion. Such a scheme is taught, for example, in U.S. Pat. No. 4,101,847, issued to A. Albanese on July 18, 1978, entitled "Laser Control Circuit." Although such an approach is useful for minimizing the second harmonic distortion generated in the laser, it does not minimize undesirable higher harmonics, and it also does not eliminate harmonic distortion generated along the transmission line.